Laminated magnetic core structure



June 16, 1964 w, c, T, JR T 3,137,832

LAMINATED MAGNETIC CORE STRUCTURE Filed Dec. 2'7. 1960 5 Sheets-Sheet l June 16, 1964 w. c. HURT, JR. ETAL 3,137,332

LAMINATED MAGNETIC CORE STRUCTURE Filed Dec. 27. 1960 5 Sheets-Sheet 2 J1me 1964 w. c. HURT, JR.. ETAL 3,137,832

LAMINATED MAGNETIC CORE STRUCTURE Filed Dec. 27, 1960 5 Sheets-Sheet 5 Fga. F7 .2 M A AW 2 2 M W2 3 2 W3 19 2e 1? 4 We? w /X/ Fgr. is.

June 16, 1964 w. c. HURT, JR., ETAL 3,137,832

LAMINATED MAGNETIC CORE STRUCTURE 5 Sheets-Sheet 4 Filed Dec. 2'7, 1960 CORE 1055' //V W197 75' PERPW/VD QEmE 33.

June 16, 1964 w. c. HURT, JR., ETAL LAMINATED MAGNETIC CORE STRUCTURE Filed Dec. 27, 1960 5 Sheets-Sheet 5 United States Patent 3,137,832 LAMINATED MAGNETIC CORE STRUCTURE William C. Hurt, Jr., Pittsfield, Mass, and Donald F.

Radke, East Chatham, N.Y., assignors to General Electric Company, a corporation of New York Filed Dec. 27, 1960, Ser. No. 78,475 4 Claims. (Cl. 336-434) This invention relates to electrical apparatus, and more in particular to improved magnetic cores for induction apparatus and methods of making same.

Electrical induction apparatus, such as transformers and the like are constructed with cores of magnetic material to provide a path for magnetic flux. The cores may be laminated structures made from stacked parallel sheets or plates of fiat, grain-oriented silicon steel. In core type transformers the core legs are surrounded by windings of coiled electrical conducting material and insulation; such windings are preferably fabricated in a circular configuration. Consequently, the winding coils define a circular opening in which the legs of the core are located. The ideal situation from the point of view of magnetic characteristics of the transformer occurs when the circular opening in the winding coil is completely filled with magnetic core material. However, practical difficulties in the manufacture of magnetic cores have generally required that the cross-sectional configuration of the core legs be either rectangular or a series of steps forming a cruciform shape. This represents a compromise between the ideal magnetic characteristics of a circular core leg and the practical difiiculties in the manufacture of cores having that configuration.

We have discovered an arrangement that permits a magnetic core to be constructed with a polygonal crosssectional area that approaches the ideal circular configuration, with the added advantages of savings in material from which the core laminations are made and savings in'manufacturing costs. The term polygonal as used in reference to a core is intended to exclude cores of cruciform or stepped cross-sectional configuration. Although the use of polygonal cores for transformers has been proposed in the past, it was previously believed that a polygonal core with more than four sides could only be fabricated on a practical basis when the core was made from a continuous or a tapered strip of metal, or when the core laminations were bent. We have discovered, however, that when the thickness of the individual :core laminations is of a small order of magnitude when compared to the over-all thickness of the core, the individual laminations may have squared edges and yet the over-all cross-sectional configuration of the core approximates that of an obtuse polygon, as defined in paragraphs that follow. Furthermore, we have discovered that when such laminations are slit from a continuous web of magnetic material having a constant width, the relative spacings'of the slits can be indexed in such a manner that all pieces slit from the constant width web can be utilized in building up the obtuse polygonal core, with a resulting elimination of wasted material.

Accordingly, it is an object of our invention to provide an improved electrical apparatus. I

Another object of our invention is to provide an improved core for induction apparatus.

Another object of our invention is to provide a polygonal induction core that can be fabricated with a minimum of scrap resulting.

Another object of our invention is to provide improved methods of making induction coresthat minimize waste of core material.

A further object of our invention is to provide a polygonal core for electrical apparatus that does not require the bending or tapering of the core material.

Patented June 16, 1964 Other objects and advantages of our invention will become apparent from an examination of the specification, drawing, and claims, and the scope of the invention will be pointed out in the claims.

Briefly stated, in accordance with one aspect of our invention, a magnetic core for electrical apparatus can be made from flat laminations of uniform thickness with each lamination being of constant width. The width of the individual laminations varies in a predetermined manner such that the cross-sectional configuration of the core is an obtuse polygon having an even number of sides greater than six. The expression obtuse polygon as used hereafter is defined as a polygon in which some sides form a plurality of equal angles, and all angles formed by the sides of the polygon are greater than An obtuse polygon is not necessarily a regular polygon.

When the core laminations are made from material hav- 1 ing a rectangular cross-section, a core in accord with our invention is considered an obtuse polygon when the thickness of each lamination is of a small order of magnitude when compared to the thickness of the core. The expression small order of magnitude, as used hereafter in the specification and in the claims is intended to mean that the thickness of each lamination is less about 3% of the thickness of the core leg.

In the drawing: FIG. 1 is an elevational view of a magnetic core in accord with our invention.

FIG. 2 is an enlarged cross-sectional view taken along the line 2--2 in FIG. 1.

FIG. 3 is a cross-sectional view on a magnified scale .of the fragment of the core leg enclosed in the circle 3 in FIG. 2.

FIG. 4 is a schematic cross-sectional view of a stepped or cruciform core leg in a circumscribing circle.

FIG. 5 is a schematic cross-sectional view of an obtuse polygonal core leg in a circumscribing circle.

.5 in juxtaposed relation.

FIG. 9 is a graph illustrating the increase in space factor obtainable from polygonal core construction.

'FIG. 10 is a graph illustrating the beneficial effect on core loss obtained from cores constructed in accord with our teachings.

FIG. 11 is a schematic representation showing a method of fabricating core laminations.

FIG. 12 is a schematic end view of a method of making core legs.

FIG. 13 is a schematic cross-sectional view of another embodiment of our invention.

FIG. 14 is a schematic cross-sectional view of still an other embodiment of our invention.

3;FIG. 15 is a schematic end view of a method of making split core legs.

Our invention will now be explained by reference to the drawing. FIG. 1 shows a magnetic core 10 for elec- The core 10 may have a plurality of legs 11 joined by nonintegral yokes 12, that extend at generally right angles to the legs according to conventional practice. The core 10 'is made from magnetic material, and preferably from flat laminations of grain-oriented silicon steel having the direction of rolling parallel to their long dimension. The corner joints between the legs 11 and yokes 12 are illustrated as being squared butt joints that overlap in alternate layers, but mitered joints may be employed without departing from the spirit or scope of the invention.

From FIG. 2 it can be seen that the core leg 11 has the cross-sectional configuration of a regular obtuse polygon of twelve sides. The sides designated S will be referred to as the first pair of sides, the sides S as the second pair of sides, and the sides S as the remaining or slanting sides. The leg 11 is adapted to be circumscribed by a conventional electrical winding coil of circular configuration (not illustrated) in the completed transformer.

The obtuse polygonal cross-sectionalconfiguration of the leg 11 is obtained by forming the core from a large number of stacked parallel laminations 13 of flat magnetic material. Each lamination is of constant width throughout its length, and all laminations may be of the same uniform thickness. When the core has mitered "corner joints, the width of the laminations will decrease only in the limited area adjacent their ends where the laminations abut; it is therefore intended that the expression constant width include laminations with mitered ends.

The first sides S are a pair of opposite parallel sides defined by the fiat outer faces of the endmost laminations 14. The second pair of opposed sides S are also parallel and are substantially perpendicular to the first sides S The second pair of sides may be defined by the aligned side edges 15 of a group of laminations of equal width 7 because each lamination has a substantially rectangular cross-section defined by its faces and side edges. The width of the remaining laminations varies in a predetermined manner, described in paragraphs that follow, from that of the laminations which define the second sides S to that of the endmost laminations defining the first sides S This enables the slanting sides S to be defined by the side edges of the remaining laminations in such a manner that the cross-section of the leg 11 is an obtuse polygon. teachings has the cross-sectional configuration of an obtuse polygon with an even number of sides greater than six in order to obtain the advantages of the construction described above.

It is evident from the large number of laminations employed and from the size of the leg 11 that successive laminations forming the slanting sides S need only be slightly different in width. It is also apparent that the cross-sectional configuration of a core leg according to our teachings is not exactly an obtuse polygon in the Therefore, a core in accord with our literal sense because the laminations are all substantially I rectangular in cross-section, and successive laminations forming the slanting sides S are of slightly different widths. Thus, when the core leg 11 is greatly magnified, as indicated in FIG. 3, the corners 16 of the laminations 13 appearto define a multitude of very small sides or steps at 90 to each other. However, the cross-sectional configuration of the core leg 11 closely approaches that of an obtuse polygon because the thickness of the laminations is of a small order of magnitude when compared with the thickness of the core leg. It follows that in a core in accord with our teachings, the cross-sectional configuration of a core leg'approaches a truly obtuse polygonal shape more closely as the thickness of the,

individual lamination becomes smaller. Therefore, the expression obtuse polygon is intended to cover core cross-sections made from rectangularly cross-sectional laminationswhose thickness is of a smaller order of magif the edges of the laminations defining the slanting sides S were cut or beveled to an angle identical to that of the sides S However, the small amount of metal added by this method isgenerally not worth the extra expense when the laminations are Within the preferred thickness range.

centerline than the n lamination will be An obtuse polygonal core in accord with our teachings provides a higher space factor than a corresponding stepped or cruciform core. In FIG. 4 a stepped or cruciform core 20 having three packets of core material 21, 22, and 23 is shown in a circumscribing circle 24. A packet of core material is defined as a rectangular stack of flat parallel laminations in which all laminations have the same width. It is apparent that the three-packet core in FIG. 4 can be made with only two diiferent widths of material, since the end packets 21 and 23 have the same width. The maximum theoretical space factor for a boltless core having the cross-section shown in FIG. 4 has been calculated as .787. This means that 78.7% of the circumscribing circle 24 would be filled with core material. A boltless core is one which does not employ bolts that pass through holes in the laminations in order to hold them together, and the maximum theoretical space factor value does not take into consideration space lost by the use of insulation, reinforcing plates, etc. that may be located within the circumscribing circle.

FIG. 5 shows a regular obtuse polygonal core 30 having eight sides circumscribed by a'circle 24 of the same size as the circle 24 in FIG. 4. The core 30 can be made from two different types of groups of laminations. The

first type of lamination groups would be a rectangular center packet 31 in which all laminations are the same width. The second type of group would be the stacks 32 and 33 in which the laminations vary in width. The stacks 32 and 33 are identical in size and shape, and are located as mirror images of each other on opposite sides of the center packet 31 at equal distances from the center line of the core. The maximum theoretical space factor for a boltless core leg having the configuration shown in FIG. 5 has been calculated as .900. This means to FIG. 6 which shows on an exaggerated scale three laminations 17, 18 and 19 which are employed along with many others to define slanting sides S The lamination 17 has a width W the lamination 18 a width W and the lamination 19 a width W The width W of the lamination 17 differs from the width W of the lamination 18 by the same amount (AW) that the width W of lamination 18 difiers from the width W of the lamination 19. This can be expressed by the equation:

with AW being a fixed predetermined amount. width of the n lamination in any stack forming slanting sides S will be W =W AW(nl), where W is the width of any lamination, W is the width of the laminatron in the same stack that is closest to the centerline of the core leg, and AW is a constant equal to the change in width between successive laminations. It follows that the width of the next lamination farther away from the and so on for each succeeding lamination.

Onemethod of determining the value of AW can be explained by reference to FIG. 7 where oneside edge of of two successive laminations '25 and 26 is shown on a The laminations 25 and 26 obtuse polygonal core. The thickness of the laminations Thus, the

' AW of the laminations.

' tion groups.

' terial.

are lamination packets of different widths.

25 and 26 is t and the lamination 25 extends beyond the lamination 26 a distance at each'side edge, since AW is the total difference in width between any two adjacent laminations. Thus a right triangle A is defined by the side edge 27 of the lamination 26, the portion 28 of the lamination 25 that extends beyond the lamination 26, and the portion 29 of the a all contangent a= 2 t since the portion 27 equals the lamination thickness t, and the portion 28 equals one-half the diiference in width It follows that AW=2t cotangent a. Thus, the formula for the thickness of the n lamination in astack forming slanting sides 8;, will be when the expression AW has been eliminated by substitution of equivalent variables. It should be noted that the value for t used in the above formula may be more than the actual thickness of the material used to make the laminations, since there are often air gaps in between successive laminations. When the laminations are very thin, the relative thickness of the air gaps becomes significant and must be determined by known empirical methods. Consequently, the manufacturers nominal thickness for the lamination material cannot always be used in the above formula. The constant AW for any specific stack of laminations defining slanting sides S will vary according to changes in such variables as lamination'thickness, slope of sides S' length of the sides S etc.

V The increase in space factor obtainable from obtuse polygonal core construction has been graphically illustrated in FIG. 8, Where the core 20 and core 30 have been superposed. The area indicated by single crosshatching at 40 shows where core material can be added by the use of obtuse polygonal core construction. Comparison of the maximum calculated theoretical space factors for the cores 20 and 30 will reveal that by employing the obtuse polygonal core 20 an increase of about 11.3% in space factor can be obtained.

It will be apparent to those skilled in the art that the cores 20 and 30 are constructed in corresponding manners, and thusthe above comparison is valid. The reason is that each core is made from two different laminapackets of two different widths and each different width is considered a different lamination group. The core 30 is also made from two different lamination groups, one

' group being the center packet 31 which has a constant width and the other group being the identical stacks 31 and 32 which have widths that vary in accord with the seen to be the number of different lamination groups used to form thecores. In stepped'cores the different groups In obtuse polygonal cores the different groups may be packets of constant width material, and pairs of identical stacks In other words, the core 20 is made from for different pairs of identical stacks. This is the basis on which a comparison is made in FIG. 9 between the space factor of obtuse polygonal cores and stepped cores. The curve I shows the maximum theoretical space factor for boltless, regular, obtuse polygonal cores, and the curve II shows the maximum theoretical space factor for boltless stepped cores. The line of numbers A indicates how many different lamination groups are employed and is the same for tall vertically aligned points on the curves I and II. The line of numbers B is the total number of packets of lamination material employed in the stepped cores represented by the curve II. The line of numbers C is the total number of sides in the polygonal cores represented by the curve I. Thus, the theoretical maximum space factor values for the cores shown in FIG. 4 and 5 can be found from the points on the curves I and II vertically above the vertical line of numbers 2, 3, and 8 because both cores have two different lamination groups, the stepped core 20 has a total of three packets, and the obtuse polygonal core 30 has eight sides.

From the curves I and II it is apparent that for any given number of different lamination groups in line A, polygonal cores have a greater space factor than stepped cores. However, the proportionate increase in space factor of polygonal cores becomes smaller as the number of different lamination groups increase. It will be apparent to those skilled in the art that when the number of different lamination groups approaches infinity, the

approaches a circle, and so the space factor of both types of cores approaches unity.

The following description is an example of commercial electrical apparatus having an obtuse polygonal core. A three-phase power transformer having a kva., rating of 3750 self-cooled had a boltless, but-lap type of core made from flat, grain-oriented, 3.25% silicon steel laminations. Each lamination had a constant width, and the core legs were twelve sided obtuse polygons in cross-section. The widest leg lamination was eleven and one-half inches in width and each leg could be circumscribed ,by a circle twelve and one-quarter inches in diameter. The laminations had a uniform thickness of approximately .014 inch. Over seven hundred laminations were stacked together to form each core leg.

As is apparent from FIG. 2, a twelve-sided core has three lamination groups. One lamination group is the rectangular center packet of constant width laminations whose side edges define the sides S The other two lamination groups are the stacks forming the slanting sides S These stacks are considered different groups because AW for the stacks forming the slanting sides which touch the sides S is different from AW for the stacks which form the remaining slanting sides and also the sides S Thus, from FIG. 9 the theoretical maximum space factor of the above-described twelve-sided commercial core leg would be about .95. This means that of a circumscribing circle would be occupied by core material. However, the space factor of the core leg on the actual transformer was slightly less than the theoretical value because the obtuse polygonal cross-section of the core was not regular.

FIG. 10 is a graph showing an unexpected reduction in core loss per unit weight of core material obtained from transformers made according to our teachings. Curve III is an empirical design curve developed over many having boltless obtuse polygonal cores in accord with our teachings; these cores also had butt-lap corner joints. Itappears from FIG. 10 that by practicing our invention,

" stant, predetermined width and uniform thickness.

parting from our teachings.

an average reduction in core loss of about 13% can be obtained'. -No satisfactory explanation for this large reduction in core loss has been found because the most significant variable in this type of loss is corner joint confstruction. -This factor can be eliminated because the corner constructions were the same for both types of cores. Furthermore, all the cores tested were made from cold rolled, grain-oriented, 3.25% silicon steel of .014" thickness. Thus the reduction incore loss obtained by a the practice of our invention is both unexpected and unexplained by present theories.

Another advantage of our core construction arises from the manner in which obtuse polygonal cores can be manufactured. We have found that Waste of core material can be eliminated by forming the individual core laminameans, such as a pair of slitting knives '72, that slits the web into two laminations 73 and 74. The web 70 may be sheared to the desired'length by shearing knives 78. The knives '72 may be laterally indexable, as indicated by the arrows '77, so that the widths of the laminations cut from the web 70 can bevaried in a predetermined manner. It will be appreciated by those skilled in the art that the apparatus schematically illustrated in FIG.

ll is merely symbolic, and other arrangements that perform the same function may be employed without de- For example, instead of indexable slitting knives 72, a pair of traversely oriented shears maybe employed to cut the web 7i) into the proper width laminations; or the slitting knife or shears may be stationary and means may be provided for indexing the web laterally.

. Waste of core material is eliminated when the width of all pieces cut from the web 70 is such that they can ,all be used as laminations in different locations in stacks 32 and 33 forming the slanting sides of an obtuse po- .lygonal core, such as the core 39 in FIG. 5. A method of accomplishing this can be illustrated by reference to FIGS. and 12.

j The eight-sided obtuse polygonal core shown in FIG. 5 can be formed from two widths of core material. The

center packet 31 of laminations having a constant width W1 can obviously be formed from core material having a width wi merely by cutting the material to the desired length. The aligned side edges of the constant width laminations would then form the opposed parallel sides 36 and 37. All of the remaining sides can be-formed by laminations cut from core materialof a single width w (FIGS. 11 and 12)'with no scrap loss, when the width W3 is equal to the width W1 of the widest lamination in the stacks 32' and 33 plus the width w of the narrowest lamination in the stacks 32 and 33.

32 and 33 are in the. shape of identical isosceles trapezoids. Consequently, means, such as the slitting knife 72 in FIG. 11, can be indexed to divide the web 70 of width W into laminations that form the identical stacks 32 and 33.' This-can be accomplished when the laminations slit from the web 70 are grouped into a pair of identical stacks, as shown in FIG. 12, with the line formed by the slits being indicated at 39. 'It is thus I apparent that the narrowest lamination in the stack 32 will'come from the same piece of material as the widest in the locations indicated in phantom in FIG. 12 where 8 p they take the same positions as the stacks 32 and 33 in FIG. 5. A further advantage of ourcore construction isbrought about because a lateraly indexable cutting means slits an endwise fed web is easily adaptable to automated production line techniques. Forexample, the indexing of the knives 72 could be controlled by a computer that is programmed to cut the constant width web 70 into laminations whose widths vary in accordance with the above-described formula: W =W AW(n'1). Cost savings could then be realized becausethe calculation of-the proper widths and proper indexing of the slitting knives would be achieved automatically by the com puter as the web of core material is continuously fed toward the slitting knives. This would permit maximum utilization of manufacturing facilities.

Another embodiment of an obtuse polygonal core in accord with our teachings is shown in FIG. 13. The cross-sectional construction of the core leg 86 differs from that of the previously'described embodiments in that the laminations forming the centermost stacks 81 *and 82 are not of the same width, so there is no rectangular center packet of constant width material. In stead all stacks are isosceles trapezoids ,incross-section, and an equal number of pairs of identical stacks are employed. The individual stacks of the respective pairs of identical stacks are oriented as mirror images of each other on opposite sides of the center line of the core,

with the widest end of each stack being closest to the center line.

The first pair of identical trapezoidal stacks 81 and r 82 each have a widest lamination of the same width W4 and a narrowest lamination of the same width W5. Thus,

the laminations forming the stacks 81 and 82 can be cut from a web of core material having a width equal to W4 plus W5 in the manner described in the preceding paragraphs with no waste of material. Similarly, the

identical isosceles trapezoidal stacks 83 and 84 can be a ,made from laminations cut from a web of core material polygonal core that eliminates the step of dividing the laminationin the stack 33, and so on, with no scrap resulting. A core with the configuration shown in FIG. 5 would be formed when the stacks 32 and 33 are placed having a width equal to W5 plus W6 without'waste of material. p

Another embodiment of an obtuse polygonal core in accord with our teachings is shown in FIG. 14. The

core is known in the art as-a split core because it step would be required. After a web 70 of predetermined width W3 has been cut into laminations 73 and '74 (FIG. 11) all laminations, including those forming the rectangular center packet, are split down their middle.

to form two identical pieces. The laminations are then grouped in the same manner as in FIGS. 5 and 12 with,

the two identical pieces forming each lamination layer being separated slightly so as to form the longitudinal gap 91. It is, therefore, intended that the term lamina- 'tion, as used throughout the specification and the claims, include a layer of laminations formed from a plurality of piecesof magnetic material aligned in the same plane. There is still another method of making a split obtuse laminations in half, as described in the preceding paragraph. This methodcan be explained by reference to FIG. 15. The eight-sided split core 90 can be made from two different widths of lamination material that need not be divided in half, just as the'unsplit eight-sided core 30 (FIGS. 5, 11 and 12) can be made from two different widths of material.

As is obvious from FIG. 15, the rectangular center packets can be made from a web of. lamination ma:

. narrowest lamination in the stacks '95 plus the Width w of the widest lamination in the stacks? In other words, the width w equals the mean width of the laminations in a pair of stacks 96. Thus when a web of material having a width W is fed past cutting means, such as the knife 72 (FIG. 11), both pieces cut from the web can be placed in one of the pairs 97 or 98 of stacks 96. Then the stacks 96 can be grouped with the stacks 95, as indicated in phantomin FIG. 15, to form the split core 90.

The method of forming the'split core 90 described in the preceding paragraph differs from the method described with reference to the. unsplit core 30 (FIGS. 5, 11, and 12) in that the width of the material used to form the stacks that define the slanting sides 8;, is equal to the mean width of the laminations in the pairs of stacks The stacks 96 can be formed 1 the spirit or scope of the invention herein disclosed, and it p is aimed in the appended claims to cover all such changes as fall within the true spirt and scope of the invention. What we claim as new and desire to secure by Letters Patent of the United States is:

1. A magnetic core having a straight laminated winding leg, said leg in cross section being an obtuse polygon having an even number of sides each, of which is a chord of a circle circumscribing said polygon, said polygon 97 and 98, rather than being equal to twice the mean width I of the laminations in a stack32 or 33, it being obvious that the pairs of stacks97 and 98 correspond respectively to the individual stacks 32 and 33. It follows that the formula for the width of the nth lamination in any stack 96 will still be W =W AW(n1), but that AW will refer to the portion where one laminaiton extends beyond the adjacent laminations at one side edge only, since the lamination side edges will be vertically aligned at the side edges that define the gap 91. Thus AW can be found by the formula AW=t cot a. By substitution,

It has thus been shown that by constructing a magnetic core according to the principles ofourinvention, waste of material can be eliminated and the advantages of. automation can be obtained for a core with an obtuse polygonal cross-section that approximates the ideal cylindrical shape. We have shown that these advantages can be obtained with cores made from fiat, parallel laminations of uniform thickness and constant width without the wasteful and expensive drawbacks of polygonal cores made from tapered-Width strips or bent plates.

It will be understood, of course, that while the forms of the invention herein shown and described constitute preferred embodiments of the invention, it is not intended herein to illustrate all of the equivalent forms or ramifications thereof. It will also be understood that the words used are words of description rather than of limitation, and

comprising at least two equal size multi-laminationpackets of isosceles trapezoidal cross sectional configuration whose non-parallel sides form four of the sides of said polygon and the shorter of whose parallel sides form two other sides of said polygon, there being at least three laniinations in each of said packets all of which laminations in each packet are of different width with the difference in width of adjacent laminations in each packet being constant.

2. A core as defined'in claim l'in which said polygon has eight sides, six of which are formed as in claim 1 and the other two of which are formed by the opposite ends of a rectangular cross section packet of laminations between the longer parallel sides of the two trapezoidal packets.

3. A core as defined in claim 1 in which said polygon has ten sides, six of which are formed as in claim 1 and the other four of which are formed by the nonparallel sides of two additional isosceles trapezoidal packets of laminations oriented with their longer parallel sides contiguous and placed between the two isosceles trapezoidal packets of claim 1, the shorter of the parallel sides of the two additional isosceles trapezoidal packets being equal in length to and respectively contiguous to the longer of the parallel sides of the isosceles trapezoidal packets of claim 1. v

4. A core as defined in claim 1 in which said polygon has twelve sides, six of which are formed as in claim 1, two more of which sides are formed by opposite ends of a rectangular cross section packet of laminations, the remaining four sides of which are formed by the nonparallel sides of two additional isosceles trapezoidal packets of laminations placed with their longer parallel sides adjacent to opposite sides of said rectangular packet and with their shorter parallel sides adjacent to and respectively contiguous with the longer parallel sides of the isoscelese trapezoidal packets of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 1,345,786 Kubo July 6, 1920 2,456,456 Somerville Dec. 14, 1948 2,550,500 Schell Apr. 24, 1951 2,991,437 Kruezer July 4, 1961 

1. A MAGNETIC CORE HAVING A STRAIGHT LAMINATED WINDING LEG, SAID LEG IN CROSS SECTION BEING AN OBTUSE POLYGON HAVING AN EVEN NUMBER OF SIDES EACH OF WHICH IS A CHORD OF A CIRCLE CIRCUMSCRIBING SAID POLYGON, SAID POLYGON COMPRISING AT LEAST TWO EQUAL SIZE MULTI-LAMINATION PACKETS OF ISOSCELES TRAPEZOIDAL CROSS SECTIONAL CONFIGURATION WHOSE NON-PARALLEL SIDES FORM FOUR OF THE SIDES OF SAID POLYGON AND THE SHORTER OF WHOSE PARALLEL SIDES FORM TWO OTHER SIDES OF SAID POLYGON, THERE BEING AT LEAST THREE LAMINATIONS IN EACH OF SAID PACKETS ALL OF WHICH LAMINATIONS IN EACH PACKET ARE OF DIFFERENT WIDTH WITH THE DIFFERENCE IN WIDTH OF ADJACENT LAMINATIONS IN EACH PACKET BEING CONSTANT. 